This invention relates to a method of charging primarily nonaqueous rechargeable batteries, and more particularly to a rapid charging method that can shorten charging time and prevent battery performance degradation due to over-charging.
A method of switching to constant voltage charging after constant current charging has been developed as a rapid rechargeable battery charging method (Japanese Patent Disclosure 3-251054 (1991)). The charging method described in this disclosure is constant current charging until battery voltage reaches a prescribed voltage. After the prescribed voltage has been reached, charging is switched to constant voltage charging to keep battery voltage from climbing abnormally high resulting in battery performance degradation. When lithium ion rechargeable batteries, which are nonaqueous rechargeable batteries, are charged by this method, constant current charging is performed until battery voltage climbs to 4.2 V. When lithium ion rechargeable battery voltage becomes greater than 4.1 V, the batteries are charged to full capacity by switching to a charging current that maintains constant voltage. This method reduces the time to full charge by making the charging current during constant current charging large. However, charging with large current requires that the size of the charging apparatus be scaled up increasing cost. It is also necessary to establish a maximum battery charging current that does not cause battery performance degradation. The time to fully charge a battery is limited by the maximum battery charging current. For this reason, methods which increase charging current have limits in reducing the time to full charge.
A battery charging method which eliminates these disadvantages is described in Japanese Patent Disclosure 2-119539 (1990). The charging method described in this disclosure is a method for rapidly charging lead storage batteries. This method charges the battery according to the voltage and current characteristics of FIG. 1 Namely, a battery is charged with constant current until a first prescribed voltage V1 is reached, and after that is charged with constant prescribed voltage V2 set slightly lower than V1. This charging method shortens the time to full charge for a lead storage battery by setting the first prescribed voltage V1, which is the final battery voltage for constant current charging, higher than the second prescribed voltage V2, which is the voltage for constant voltage charging.
The method described in this disclosure can shorten charging time compared with methods that perform constant voltage charging from the start. This is because the amount of charging with constant current from the start until the first prescribed voltage V1 can be made greater than that for constant voltage charging from the start with gradually decreasing current over the same time period. Consequently, the overall charging time can be reduced by this method. Further, in this type of charging method, charging time can be reduced by increasing the final battery voltage V1 for constant current charging. However, when first prescribed voltage V1 is increased, nonaqueous rechargeable battery performance is reduced due to secondary reactions within the battery.
Degradation due to secondary reactions can be prevented by constant voltage charging from the start at the second prescribed voltage V2. However, this has the disadvantage of increasing the overall charging time. Consequently, this charging method cannot mutually satisfy the opposing conditions of reducing charging time and preventing battery performance degradation. The present invention was developed to further solve these problems. It is thus a primary object of the present invention to provide a battery charging method that can shorten charging time and drastically reduce battery performance degradation due to over-charging.